For those of us in northern climates, strawberries are the first tangible evidence that summer has truly arrived. These fragile berries, filled with water and sugar, are beloved both by people and by the kajillion-strong microbe population. To enjoy strawberries, you've got to be quick, because the invisible kingdom of bacteria and molds is just as eager to take a bite. Every moment that your haul of berries sits on the counter, enzymatic decay makes them increasingly susceptible to microbial colonization. It's a race against time that determines who will consume the berry first: you, or the microbes.

Your best defense, aside from eating them all right away, is preservation, but "preservation" is a bit of a misnomer; it's really extension. All preserved foods have a shelf life, and eventually they, too, will degrade to the point of becoming inedible. In the end, all we're trying to do is buy ourselves some time, and the better we understand the science that underlies preservation, the more time we can buy.

The Antimicrobial Tools of the Preservation Trade

Most microbes thrive in environments similar to the ones that support human life. They require the same basic elements: water, food, and oxygen.* They also have environmental preferences for certain temperature and acidity levels. It helps to visualize a microbe as a wealthy high-society matron in Boca Raton—the environment has to be just right for her to thrive. Too hot, too cold, too dry, too acidic, too salty, too much anything, and the microbe will die. And, just like Boca retirees, microbes have varying degrees of hardiness; the more delicate the microbe, the less it takes to kill it off. If we don't want these old-timers...er...microbes around, we can change the chemical and physical conditions of the environment, erecting barriers that block them and increasing the edible life of a food.

* There's a pesky class of microbes that thrive in anaerobic (oxygen-free) conditions. Clostridium botulinum is one of the deadliest, and it loves low-oxygen, low-acid environments. Botulinum toxins can kill you. In a later article, we'll talk about how to keep them away with the help of acid—because no one should ever have to die because someone chose to ignore science!

There are several approaches we can use to do this, often in combination. Pickling, for instance, is about creating a high-acid solution—too high for microbes, but still tasty for us. You can pickle strawberries, especially underripe ones, but it's not the most common approach.

Pressure canning, meanwhile, is about raising temperatures to a point so high that nothing will survive. Sure, we could expose our strawberry harvest to the extreme heat of a pressure canner, but to what end? The 240°F (116°C) temperature would kill any and all microbes, but it'd also turn the strawberries into a jar of overcooked mush. It'd be like launching a nuclear bomb when a BB gun would have done the job. Freezing foods is on the opposite end of the temperature spectrum, as extreme cold prevents microbial growth. If you have the freezer space, that's certainly an option for keeping an abundance of ripe strawberries.

Gravlax relies on salt and sugar to lightly cure salmon. It's not enough for long-term preservation, but the salt and sugar lower the fish's water activity enough to buy you a few extra days of freshness.

We'll discuss acids and temperature in future articles on the science of preservation, but today we're interested in yet another essential method: reducing the available water in the food. Any time a recipe involves preserving a food using lots of salt, sugar, and/or dehydration, it falls into this category. Salt-cured meats and fish, fruits poached in sugary syrups, jams, jellies, and dehydrated snacks like jerky and fruit leather all operate under the same basic principle: drive off or tie up enough of the available water in a food to make it inhospitable to microbes.

Hell or Low Water

All living matter is an assemblage of cells, and those cells are made from molecules. Those molecules are made of elements, and each element is built from atoms. Fruits, vegetables, and meats are constructed primarily from the elements carbon, hydrogen, and oxygen, which are mostly arranged into molecules of water, carbohydrates, protein, and fibers. Of course, you don't need to know the exact chemical makeup of every single thing you eat to be able to prepare it appropriately; for now, the important thing to keep in mind is that every food contains water.

From a food safety standpoint, though, what matters most is not the absolute water content of a food, but its water activity. The technical definition of "water activity" quickly gets us into scientific-formula territory, so for our purposes, we can think of it more simply as the amount of water that's available to a molecule—and that includes all the molecules that make up a living organism. Since all organisms need water to live, we can prevent them from contaminating our food by reducing the amount of water that's available to them—that is, by reducing the food's water activity.

Think of it this way: Imagine that all the water molecules in a piece of food are just people in a room. And then think of a microbial pathogen as a really annoying guy who's always trying to get into the room and make trouble. One solution would be to get as many people out of the room as possible before the annoying guy arrives; that, in essence, is dehydration. With fewer people in the room, he has fewer people to bother. But let's say that, for a variety of reasons, it's not possible (or desirable) for every last one of those people to leave before the guy comes in. In that case, we need another strategy. The people in the room could, for example, occupy themselves—perhaps by putting on headphones to listen to music, or reading a book. They'd be busy, and therefore unavailable to the annoying guy. He'd eventually die of boredom, or wander off in search of another room full of more available people.

The key thing to understand is that the room had people in it the whole time, just as food always has some amount of water in it. They just weren't available to the annoying guy. But, of course, this only works if there are enough books and headphones in the room for everyone to use.

The question, then, is what the book and headphone equivalents are in the world of food. There are many potential ones, but two of the most powerful are salt and sugar. Add either (or both) of them to the food and they'll bond with the water molecules, tying them up and making them unavailable to any microbes that come around looking for a nice place to stay. Add enough salt and sugar, and there won't be sufficient water molecules available for the microbes to survive, even though the food still contains plenty of water.

From a chemical perspective, it's helpful to think of this riddle: When is water not really water? When it's salt water.

Strawberry Bonds

Sugar molecules bond with the water in the strawberries to form a syrup and lower the fruit's water activity.

Let's get back to our strawberries. Thus far, we know that we want to reduce the water activity of the strawberries enough to make a microbe's survival damned hard. But how much is enough? Measuring water activity in a food requires complex and expensive laboratory equipment that most of us don't have.** For most of us, the best we can do is rely on tested recipes or, even better, formulas that can help ensure our strawberries are in the safety zone.

** Scientists have methods for measuring water activity, which they express as a value (aw). Pure water has an aw of 1.0. The ideal target for preservation is a water activity of 0.85 to 0.80 (aw). But, again, you don't need to know how to measure or calculate this as long as you have a good recipe to follow.

If we were to simply cover our strawberries in sugar, the berries' water would begin to leach out, then combine with the sugar to form a syrup. The syrupy strawberries have just as much water as the berries did before the sugar was added, but their water activity has been reduced, all thanks to the sugar molecules bonding with the water molecules. That alone, a process cooks call maceration, buys you a few more days of usability.

Add heat to the sugar-covered strawberries and the water activity is reduced even further through evaporation (and, as a result, dehydration), buying you even more time. Most preserved foods rely on such combinations of chemical and physical barriers to ensure no pathogens can grow. Even the simple act of sealing the sugared and cooked strawberries in a clean glass jar, then placing them in the fridge, is a line of defense—one more wall that makes it difficult for ambient microbes to move in.

But we want to take our preservation a step further by making jam, which will get us even more usable time out of those strawberries.

Preservation in Action: Strawberry Jam

Flex Your Pectins

Strawberries and sugar alone aren't enough to make jam. You need enough pectin, a natural fruit fiber, for the jam to thicken properly.

We've finally made it to the fun part of food preservation, where we'll master water activity by actually making some strawberry jam.

In our above example, we tossed fresh strawberries with sugar, then cooked the two together. One might assume that we made jam during that process, but we didn't. While definitions can vary, most would agree that for jam to be jam, it can't just be cooked fruit in a sugar syrup. It requires a thickener called pectin, a natural fiber in fruits and vegetables that keeps the outer peel or skin firm and intact.

Some fruits, like apples and oranges, have more pectin; some, like strawberries and raspberries, have less. Strawberries, which have very little pectin, will never set into a thick jam on their own—at best, they'll cook to a thickened syrup. Old European methods of jam-making resolve low pectin levels in a berry jam by tossing an apple, quince, or orange into the mix. Some old recipes don't even say to do this, as it was just assumed that a home cook would know as much.

Today, we can add either homemade pectin (super-thick apple jelly, for instance) or commercial pectin to our strawberry/sugar mixture to account for its absence in low-pectin fruits like strawberries. Commercial pectin is derived from citrus peels and refined into a powder, and it comes in a couple of different forms.

A "regular" pectin is chemically designed to thicken when combined with heat and sugar. A "low-sugar" pectin undergoes additional refinement to work with lower amounts of sugar. Low-sugar pectins also increase the overall acidity of a jam, which erects yet another barrier to microbial growth. They usually contain a small amount of dextrose, which is a hyper-absorptive sugar, and sodium citrate, an acidifier. Using it requires a catalyst, in the form of an acid (from lemon juice) or calcium water. Regular pectin requires no additional catalyst.

I love low-sugar pectin, especially the Ball and Sure-Jell brands. It allows flexibility with the type and amount of sugars used. (This includes fruit juices and the artificial sweetener Splenda.) Purists may balk at the idea of adding dextrose and sodium citrate to a jam, but it's the dextrose that ensures sufficiently reduced water activity, even with lower levels of sugar in the recipe as a whole.

One word of warning: The Pomona's brand of low-sugar pectin works a little differently from Ball and Sure-Jell, relying on calcium water to activate the pectin with little to no additional sugar. The method of incorporating it into a jam is different from other low-sugar pectins, so you can't easily substitute it unless you know how. Even more importantly, Pomona's doesn't really reduce the water activity to a safe level so much as it acidifies the jam. Personally, I don't use Pomona's Pectin because, quite frankly, it makes me nervous. Jam is safely preserved only once we've adequately reduced the water activity; acidification alone doesn't do it.

Ready...Set...

Many old-school jam recipes require cooking the fruit-and-sugar mixture for a prolonged period of time at 220°F (104°C). This is meant to do two things: reach an 80% water activity (the point at which water activity is reduced enough to prohibit pathogen growth) and get the jam to jell sufficiently. (Candy-makers will recognize that 220°F temperature as just below the "thread" stage.) This is the method used in many European recipes. Here's what's happening: Sugar bonds with the fruit's water molecules, reducing the water activity, while heat evaporates more of the water, further reducing the water activity and causing sugar and carbon molecules to re-chain into tight bunches, which results in thickening. Achieving and maintaining the ideal 220°F temperature requires vigilance and patience. It's a putzy job of stirring, adjusting heat, waiting, and repeating. Those who do use this method should use caution—if you allow your mixture to overheat, the batch can quickly transform into something other than jam. It might turn into candy, or it could just scorch and burn.

The dextrose in commercial pectin, whether regular or low-sugar, provides a scientifically sound shortcut that allows you to avoid that whole process. When you add pectin to your fruit-and-sugar mixture, that dextrose kicks in to rapidly bond with available water molecules, helping you reach your desired water activity almost immediately—no need to cook the jam for a long time to get there. Bringing the mixture to a hard, rolling boil for one minute does the trick and is much easier than the method required if you don't use a commercial pectin.

How do you know if you've got it right? How can you tell if your jam is set and not just syrup? You're looking for a consistency at room temperature that is, in a word, gelatinous. A good jam set, as it's called, will remain intact when scooped, but can also be spread thinly, without seeming watery. It shouldn't have any lumps, save for the tasty pieces of fruit, nor should it be so thick that you can stand a knife up in the jar.

Because fruits contain varying amounts of sugar and pectin from one batch to the next, the set may differ between batches, even when you're using commercial pectin. And that's okay. The beauty of homemade jams is the subtle differences that exist between batches and cooks. Homemade jams have—dare I say it?—a terroir, like wines. The wild summer strawberries of northern Minnesota will taste somewhat different from those in Grandma's Alabama berry patch. The jams made from those berries will reflect their unique geographic and territorial markers.

A majority of the frantic phone calls I get from novice jam-makers communicate variations on "My jarred jam didn't set! Now what?" The solution is to figure out if the jam is set before you pour it into jars or containers. I use what we've dubbed the "Moses Test," because your goal is to "part the sea": Take a teaspoon of the hot cooked jam out of the pot, and place it on a small dish. Let it completely cool—like, seriously cool, to room temperature. Five minutes is good. You can use that time to prepare your jars or containers. Now run your finger through the middle of the splotch of jam. Do the edges run back together to the middle? If they don't, then you've successfully parted the sea, and your jam is set.

Formula One

Once you understand the jam formula, you can safely invent your own recipe.

We now have the key ingredients for our strawberry jam: strawberries, sugar, pectin, and lemon juice (if you're using low-sugar pectin). The cool thing is that once we learn a very basic formula—and because we understand the science behind water activity and preservation—we can break free of any specific recipe.

What's the formula? For a strawberry jam using regular pectin: 4 cups strawberries + 4 cups sugar + 1/3 cup pectin. For a strawberry jam made with low-sugar pectin: 4 cups strawberries + 2 cups sugar + 2 tablespoons lemon juice + 1/3 cup low-sugar pectin. In fact, that's not just the master formula for strawberry jam; that's the master formula for most jams. Four cups of almost any fruit, when combined with sugar and pectin (again, plus the lemon juice, if you're using low-sugar pectin), will, in the vast majority of cases, yield a beautiful jam. Sure, there are some exceptions here and there, and you can go too far out of bounds in an attempt to be wildly experimental, but for the most part, this formula works. The variations in natural sugar and pectin content from one fruit to the next are slight enough that the overarching ratio will still lead you to success.

Since we know what each of these ingredients is doing to the other, we can swap and change them around. Mix in oranges. Or ginger beer. Or red wine. Don't want to use white sugar? Use brown sugar. Or maple syrup. Or hickory syrup. Or Splenda (but only if using low-sugar pectin, since you'll need the added dextrose to make up for the missing sugar). As long as you keep the ratios of the formula intact, you can add herbs, spices, and other flavorings without affecting your overall success in preserving the strawberries as a jam. How best to create your signature strawberry jam? Using the four cups of berries as the base measurement, make your substitutions accordingly—say, three cups of strawberries plus one cup of ginger beer. Or two cups of strawberries and two cups of wine. It won't affect the result if your substitutions are liquid or solid. Some added ingredients, like candied ginger, may already be "sugared." In those cases, you have a choice—proceed with the formula for a sweeter batch, or reduce the total sugar amount to compensate. The water activity will still be sufficiently reduced because the sugar is still there, just coming from a different source.

In the classes that I teach, this is the point when I see faces light up. They're now thinking of all the tasty and silly and personal combinations they can create. This is the alchemy of jam-making. Did you know that strawberry and horseradish complement each other magnificently? The cooking process tempers the horseradish, and, when married to the sugar and strawberries, it produces a jam that will change every roast beef sandwich in your life. Using chopped candied ginger and candied orange peels together increases the sweetness, but adds a lovely toothsome quality to strawberry jam. If you follow your imagination and palate, the possibilities are endless. If you don't own it already, check out The Flavor Bible by Karen Page and Andrew Dornenburg. It's a nerd's food book—an encyclopedic listing of every food and which other ingredients will complement it. It's a great resource for the kitchen alchemist.

Jam Session

The final step is to ask yourself what your goals are for your preserves. Do you need the jam to last for months on end in a cellar? Then you'll need to heat-process it on top of everything else. If you merely want to store the jam in the fridge for several weeks, you can skip that step.

Heat-processing the jam, a technique I'll cover in more depth in a later article, involves submerging a lidded jar into a bath of boiling water (or placing it in a pressure canner) for a prescribed amount of time, during which any remaining oxygen is driven out and the jars will be sealed against new microbes. Like I said at the beginning, many preserved foods call on a combination of microbial barriers for guaranteed success.

At this point, you're ready to erect that first barrier—low water activity—with the help of salt, sugar, and dehydration. It's a key tool in the multipronged approach to blocking microbes from our food and keeping it edible for much, much longer.